1. Field of the Technology
The present invention relates to the techniques of combining Circuit Switched bearer with IP multimedia subsystem services (CSI), more particularly to a method and a system for ensuring communications of a CSI terminal.
2. Background of the Invention
The traditional telecommunication network includes a Circuit Switched (CS) domain and a Packet Switched (PS) domain, performing CS services (including voices, data services of CS type, faxes, etc.) and PS services respectively. At present, based on the emerging trend of the Internet and the telecommunication networks, the 3rd Generation Partnership Project (3GPP) introduces an IP Multimedia Subsystem (IMS) based upon the original PS domain. The object of the IMS is to provide multimedia communication experience for users according to individuating user data, shielding access methods of users.
Both signaling and media streams of the IMS are born by the PS domain. Since in the PS domain, the network resources are used in a shared mode, which is different from the dedicated mode in the CS domain, for real-time services, e.g. voice, the transmission quality will be obviously degraded when the PS bearer instead of the CS bearer is adopted, and it may be impossible to satisfy the service requirement of operators and users. Therefore, 3GPP provides the CSI service which combines the CS bearer with the IMS service, i.e. multimedia services are implemented by combining the PS bearer and the CS bearer, while the CS bearer is still adopted for real-time services such as voice, and the PS bearer is adopted for other non-real-time services such as pictures and messages, and meanwhile, the IMS provides the uniform control.
FIG. 1 is a schematic diagram illustrating an implementation of the CSI service which combines the CS bearer and the IMS service. As shown in FIG. 1, the CSI service provides the same service experience as that of the multimedia service provided by the IMS to the end user, but the transmission delay for the real-time media component is greatly reduced by using the CS bearer.
At present, 3GPP TS 23.899 provides a logical function architecture for implementing the CSI service, which is shown in FIG. 2. As shown in FIG. 2, the architecture includes a Circuit Bearer Control Function (CBCF), a Circuit Bearer Originating Function (CBOF), and a Circuit Bearer Terminating Function (CBTF). The CBCF determines and controls the establishment of the CS bearer between the CBOF and the CBTF. The above three functions can be implemented in existing IMS functions, e.g. a User Equipment (UE), and an Application Server (AS). The detailed location can be negotiated during the establishment of a session. The CBOF or the CBTF can also be a Session Initial Protocol User Agent (SIP UA), thus establishing the CS bearer between the CBOF or the CBTF and the peer side means establishing the CS bearer between a CS/IMS interworking gateway representing the CBOF or the CBTF and the peer side.
The 3GPP TS 23.899 provides two modes for the establishment of the CS bearer: end-to-end mode and end-to-gateway mode. In the end-to-gateway mode, the bearer is established between a terminal and a gateway, and the CS bearer can be established in either a client-to-network direction or a network-to-client direction. The choice is independent of the session direction. In addition, the 3GPP TS 23.899 also provides principles that the CSI should follow and problems to be solved, e.g. only the IMS and CSI-capable UEs can be impacted when the CSI service is implemented, and other network entities, UEs, and interworking signalings between the IMS and other networks shall not be impacted. It is possible to perform charging correlation of CS/PS/IMS, and ensure the consistent of the end-user charging.